Análisis de los defectos encontrados

As you learned, a spark plug is designed to allow a voltage to jump across a gap, producing a spark that ignites the engine’s fuel. Four-stroke engines contain one spark plug for each cylinder. An external

FIGURE 16—This

view of a spark plug is shown inFigure 17A. The basic parts of a spark plug are shown inFigure 17B.

The metal section at the bottom of the spark plug is called the shell. The top section of the shell is molded into a hexagon shape that fits into a wrench or socket. Thus, a wrench or socket can be used to install or re-move a spark plug. The lower section of the shell is threaded. Remem-ber that a spark plug screws into a hole of the cylinder head. The threads on the bottom of the spark plug mate with threads inside the hole in the cylinder head.

A spark plug has two metal electrodes or terminals. The metal elec-trodes are conductors through which current flows. One electrode runs through the entire length of the spark plug. This is called the cen-ter electrode. The second electrode is connected to the threaded part of the spark plug. This electrode is sometimes called the side electrode or the grounding electrode. The grounding electrode bends around so that it’s very close to the end of the center electrode. The small air space be-tween the two electrodes is called the gap.

The top end of the center electrode connects to the terminal nut of the spark plug. When the spark plug is screwed into the cylinder head, the terminal nut is connected to the spark plug wire.

FIGURE 17—Figure 17A shows an external view of a typical spark plug. Figure 17B shows the parts of a spark plug.

The high voltage produced by the ignition coil travels through the spark plug wire and enters the spark plug through the terminal nut.

The electricity then flows down the spark plug through the center elec-trode and jumps across the gap from one elecelec-trode to the other to pro-duce the spark.

Different plugs have different types of electrodes. In some plugs, the center electrode is made of an alloy of copper and steel. Other plugs have electrodes that are made of a platinum alloy. Platinum-alloy elec-trodes operate better under high temperatures and burn off combus-tion deposits at lower temperatures. The various spark plug manufac-turers usually indicate what type of electrode the spark plug is equipped with. The best advice as far as choosing a particular type of spark plug is to use the plug that’s recommended by the vehicle manufacturer. This information is usually listed in the vehicle’s service manual.

In some spark plugs, a small ceramic element is placed in the center electrode. This element acts as a resistor, preventing the spark plug from interfering with radio frequencies. When a spark plug fires, it sometimes interferes with the radio. This interference causes a pop-ping noise in radios, televisions, and in some types of communication systems. The resistance element in the plug helps to prevent this inter-ference.

The shape of the grounding electrodes in spark plugs varies. Most grounding electrodes bend and extend over the entire width of the center electrode. This is sometimes called an automotive gap spark plug.

However, in some plugs, the grounding electrode is split to form two separate grounding electrodes. This type of spark plug is sometimes called a split-fire or split-electrode spark plug. The manufacturers of these plugs claim that the split-type electrode offers better engine perform-ance and fuel economy. However, this is a matter of opinion; some technicians believe the split electrode does provide benefits, while oth-ers feel it offoth-ers no performance advantages. Again, use the spark plugs recommended by the manufacturer. An automotive gap and a split-electrode spark plug are shown inFigure 18.

AUTOMOTIVE GAP

shows the end of an automotive gap spark plug. Figure 18B shows a split-electrode spark plug.

important to the correct operation of the spark plug. If a gap is too narrow, the spark produced is weak and the ignition is poor. In contrast, if the gap is too wide, it’s difficult for the electricity to jump the gap. This condition also results in a weak spark. Therefore, you can see that the width of the gap is a very important factor in ignition system performance.

The body of a spark plug is encased in a porcelain shell. Porcelain, a china-like substance, is used for the shell because it’s an electrical insula-tor—it doesn’t conduct electricity. This porcelain insulator electrically iso-lates the voltage inside the spark plug. The spark plug’s manufacturer and identification number are usually printed on the porcelain insulation.

Note that the porcelain covering is ribbed. The ribs extend from the terminal nut to the shell of the plug to prevent a condition called flashover. In flashover, current jumps or arcs from the terminal nut to the metal shell on the outside of the plug instead of traveling down through the center electrode.

You learned earlier that the spark plug wire is connected to the spark plug by a metal connector that fits down over the plug’s terminal nut.

A typical spark plug wire connection is shown inFigure 19. Note that this connector has a rubber boot that seals out dirt and moisture. The boot also prevents the high voltage from jumping out to the cylinder head instead of flowing down to the spark plug electrode.

If you look quickly at a group of spark plugs, they may all look alike.

However, spark plugs are manufactured with minute differences that affect their performance. Each type of spark plug is identified by a spe-cific manufacturer identification number. When you replace a spark plug, always use the same type of plug.

Now, let’s discuss some of these different spark plug specifications.

The first specification is called reach. The reach of a spark plug is the length of the metal threads at the end of the plug. The reach of a spark plug is shown inFigure 20.

SPARK PLUG WIRE

RUBBER BOOT

SPARK PLUG FIGURE 19—The insulated

connector on the end of a spark plug wire fits down around the spark plug’s terminal nut as shown here.

The correct spark plug reach is essential to proper engine operation. If the spark plug reach is too long, the threaded part extends down into the combustion chamber and hits the piston each time it rises, which seriously damages the engine. If the reach is too short, the spark occurs too high in the cylinder head. This causes the air-and-fuel mixture to burn too slowly in the combustion chamber and delays the start of the power stroke. The delayed power stroke causes a loss of power and makes the engine difficult to start.

Spark plugs also differ in terms of how much heat they can withstand.

Heat from the fuel combustion process is absorbed by the spark plug during engine operation and is conducted upward through the plug.

Combustion temperatures normally range from 1,000 to 1,500 degrees Fahrenheit. Thus, a spark plug must be able to withstand these temperatures.

Each spark plug has a heat range. A spark plug’s heat range deter-mines, to a large extent, engine performance under different conditions and speeds. A heat range classifies a spark plug according to its ability to transfer heat from the gap end of the plug to the engine’s cooling system. The rate of heat transfer is controlled by the length of the insu-lator tip, as shown inFigure 21.

FIGURE 20—The length of the threaded area of a spark plug is called the reach.

FIGURE 21—A spark plug’s heat range is a measure of the plug’s ability to transfer heat. The rate of heat transfer is controlled by the length of the insula-tor tip. The hot plug in Figure 21A transfers less heat than the cold plug shown in Figure 21B.

plug, the center electrode is more isolated from the shell and the cylin-der head. Therefore, a hot plug tends to retain its heat. Cold plugs have shorter insulator tips than hot plugs.

Spark plugs are made in several heat ranges to suit different engines and different operating conditions. A spark plug with the correct heat range must be installed in an engine. For instance, a cold plug should be installed in an engine that has high combustion temperatures. A hot plug should be installed in an engine with low combustion tempera-tures. If a hot plug is installed in a hot-running engine, the spark plug may overheat. If a cold plug is installed in a cool-running engine, heavy carbon deposits form on the electrodes, making it difficult for the spark plug to fire. When the plug is in the correct temperature range, the heat from combustion burns the byproducts of combustion off the electrodes and keep them clean without causing them to over-heat.